Malignant lymphomas and leukemias are related to maturation stages of distinct subpopulations of hematopoietic or lymphoid cells as defined by immunocytological methods. We first describe various monoclonal antibodies, and the methodology employed in our investigations and report on recent developments in the standardization of these reagents (part 1). The maturation sequence of normal lymphoid cells and their precursors in the bone marrow, thymus and the peripheral lymphoid organs are discussed in part 2. Typical immunomorphological findings in Non Hodgkin Lymphomas (NHL) are summarized in respect to lymphocytic NHL (CLL, lymphoplasmocytoid NHL, hairy cell and prolymphocytic leukemia, some lymphomas of peripheral T-lymphocytes), to NHL of follicular center cells (centroblastic-centrocytic, centrocytic NHL), to "large cell" NHL (centroblastic, immunoblastic NHL, large cell NHL derived from T-lymphocytes or "lymphomas" of macrophage origin) and to lymphoblastic NHL (derived from T-lymphocytes, pre-B lymphocytes and Burkitt-type). Findings in multiple myeloma are also summarized in part 3. Immunocytological features of the normal Myelo-, Mono-, Erythro- and Thrombopoieses are discussed in part 4. The reactivity of some monoclonal antibodies with precursor cells of these cells (CFU-GM, BFU-e and CFU-e, CFU-M) are also described. Finally we summarized the immune phenotype of acute leukemias (part 5) in respect to acute lymphoid leukemias (cALL, T-ALL, O-ALL, B-ALL), to acute non lymphoid leukemias (M1-M6 type according to the FAB-Classification) and to blastic stages of chronic leukemias.